The appearance of ducts and vents on the bonnets / hoods of cars often denote sporty or performance intentions. NACA ducts and hood scoops feed air into the intake or through top-mounted intercoolers and rear facing vents extract heat and encourage additional ventilation through the front radiators, lowering the pressure inside the engine bay. Several generations of the Mitsubishi Lancer Evolution are good examples of this, some featuring both intakes and extractors and all are designed to be functional.
Unfortunately, they are often pasted on as a questionable cosmetic enhancement by consumers, though some OEM production cars are just as often guilty of this. Gluing on a fake hood scoop does exactly the opposite of performance enhancement. But I’m definitely all for attaching a functional heat extractor vent that just happens to look good, too.
Knowing the potential benefits of installing a vent, I cut into my precious Roadster hood several years ago and installed a set of Flyin’ Miata aluminum louvers. I roughly followed their recommendations for vent placement for maximizing airflow in the lower pressure regions of the hood, but I opted for stainless screws instead of rivets for ease of removal should I decide to repaint in the future. I also drilled holes through the hood support ribs instead of removing them completely to keep rigidity.
It turns out the vent functioned really well. While I didn’t have a water temperature gauge to measure engine bay temperatures at the time, I estimated temperatures before and after the modification. After venting, I found that the heat is kept fairly moderate and consistent, even when running hard at a local track or Auto-X. There is a drawback, however: there is a slight smell from the engine when I am idling or driving slowly through traffic with the windows down.
I picked up these vents off of Yahoo! Auctions, which is much more popular than eBay in Japan, by far. I initially had no idea where they originally came from, as they appear to be from some OEM application judging from the mounting screws on the underside. After scouring the internet for literally hours (while watching Netflix), I finally found the source of the vents: a relatively rare 1990-1994 Nissan Pulsar GTI-R, an awesome WRC-homologated, AWD turbo rally car. While they are a bit large, I think these 90s vents will fit well with the old-school feel of the car, providing both a rally look and additional cooling.
Before I put the vents on, some testing is in order. While there is plenty of pressure numbers available for the Roadster on the internet, but I couldn’t find real testing results for this generation of Alto. Simply judging vent placement by various users on Minkara didn’t tell me how much they actually tested it, other than “It looks cool” and “I think it works.”
This is commonly called the “tuft test” and it involves taping some yarn to various areas, taking a drive, and recording the results. The motion of the thread can tell us two important things: airflow turbulence and air pressure around the various surfaces of the car. The information from this will help me determine the best place for the vents. (Just as an aside, I actually made a mistake on the tufts, as they should've been cut shorter as to not touch the other masking tapes. However, it should not have affected my findings much.)
I was honestly a bit surprised from the videos I took of the yarn. It looks like the hood has major airflow around more surfaces of the hood than my Roadster did with similar testing, perhaps due to the rather angled hood surface. As with most cars, there are some lower pressure areas in the middle area of the hood, away from the windshield, front bumper, and fenders.
Along with testing the hood, I also did a bit of testing along the fenders. As with most cars, the air directly behind the wheel is extremely turbulent, but fairly smooth and consistent near the top of the fender. There could be a lot of improvement in this area… if you didn’t already guess, I have plans for the fenders soon.
Before I get into cutting into the hood, I wanted to make sure critical components under the hood are well protected from the elements like the heavy rainfall we experience in Japan. I already designed the heat shield for my HKS Super Power Flow intake with the idea that I would eventually install hood vents, hence the extra large piece of aluminum effectively covering more than half; well past where I would install the hood vent in an effort to prevent waterlogging the engine. However, the left side has a critical component not covered yet at all: the battery.
Obviously, as aluminum is an effective electrical conductor, I have to use a plastic or polymer of some sort. At the hardware stores around Japan, I can find large quantities of different plastics like acrylic board and PP (polypropylene). While acrylic and PP are heat resistant up to around 100°C and 130°C (212 °F and 266 °F) respectively, I wanted to make sure it would not melt or warp in the heat of the engine bay over prolonged periods of time.
After scouring a massive number of hardware stores in Japan, I finally found Royal Pro sold large sheets of polycarbonate at a maximum thickness of 3mm. I would have preferred 5mm for additional stiffness, but this will be adequate since it will not see any major stresses. Polycarbonate, or commonly referred to by the company name Lexan, is a super strong, flexible, heat and shatter-resistant clear polymer, often used as windows on race cars. I wanted to use this over other common plastics for those above qualities, especially as it is resistant to around 147 °C (297 °F). If the cooler side of my engine bay ever got to those temperatures, I’d have a lot more to worry about than melting plastic.
However, since this is all in the testing phase, I didn’t want to dive in and put down 7000￥ ($64 USD) for a sheet of polycarb. I found a very similar material with heat resistance up to 120 °C (248 °F), polypropylene (PP), at the 100￥ (dollar) store. At that price, I can do a lot of experimenting, so I opted for a 2mm thick sheet for now. Plus, its a reasonable size that I can forego with cutting down significantly.
I figure this position would offer the most coverage of the battery and electronic connectors. A few holes, cuts, and a bit of rubber door edge protector at the edge it touches the hood, and the protector is finished.
I just decided to piggyback off the existing battery tie down mount for a simple, clean installation. I used threading connectors to attach the top screws with the battery tie downs and two water-resistant foam and stainless washers to prevent leakage down through the holes. This should be more than adequate to run off excess moisture and prevent battery shorts.
I will pull out the drill next time.